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SAE vs. Metric SizesFraction to Metric

I'm sure you sit around on your day off converting fractions to metric numbers just for fun. No? You don't? I'm shocked! So who cares that American standard tool sizes do not exactly fit metric nuts & bolts? They'll work, right? No, they won't. If you want to work on a non-American bike, you will need to be familiar with metric sizes.

If you are the type of mechanic who uses a butter knife to remove screws because the tool box is all the way over there stop reading. I can't help you.

Standard American tools are often called SAE after the Society of Automotive Engineers who developed the size standards in the early 1920's.
SAE tools are measured in fractions of one inch.

Machines designed outside the US use the metric system for
measurements. Metric tools are measured in millimeters.
Japanese,
British, European, etc cycles will have all metric bolts. Do not use standard sockets on metric bolts.

If you are new to bikes or just getting started in their repair, the chart below will help estimate of the size tool needed, if you are not familiar with metric sizes. If a metric bolt head looks like a 5/16" it is probably an 8mm. Mechanics should invest in a good micrometer for their toolbox. A model you can switch between Std & metric measurements. A tape measure will not get you by for long. Also, they are useful if you need to do a valve adjustment.

This chart lists fractions and their decimal and millimeter equivalents.
The millimeter measurement column is not rounded up or down.
We have only included common sizes here.